Hello- Thank you for asking the question. I have over 30 years of experience working with individuals, couples and families & am happy to reply.
Thank you for this interesting question and I am sorry to hear about your sons problems.
The best way to get a prescription is for your son to receive a letter from a psychiatrist stating that he has a mental disability i.e. PTSD that requires such assistance.
Here is information outlining the criteria:
In case you didn't know there is also an organization that provides in depth information about going through this process as relates to Psychiatric Service Dogs and Emotional Support Animals.
It is very important to educate yourself about this and you should find answers to every question at the following site:
Should you have additional questions of me, feel free to respond and I will be happy to reply.
Otherwise, I wish you the and your son the very best.
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